Phenotyping and genotyping of composite lymphoma with Ki-1 component

A case of composite lymphoma consisting of an anaplastic large-cell Ki-1 lymphoma and a small-cell follicular lymphoma was found in the splenic hilar lymph node of a 66-year-old woman. The Ki-1 lymphoma showed monoclonal IgM-λ and CD 20, CD 74, and CDw 75 antigens by immunostaining and CD 19, CD 20, CD 22, and lambda antigens by flow cytometry. The follicular lymphoma also showed monoclonal IgM-λ, and CD 20 and CDw 75 antigens but not CD 74 and CD 30 (Ki-1) by immunostaining. Flow cytometric analysis of the follicular lymphoma component was not conclusive, as it was impossible to separate the neoplastic from the normal small B lymphocytes. Ki-1 lymphoma usually is seen in childhood and is mostly of T cell origin. It is, therefore, unusual to find Ki-1 antigen component in a composite lymphoma of B-cell origin in an adult. However, there has been evidence to suggest that B-cell Ki-1 lymphoma may be related to follicular lymphoma. Thus, our case may represent a follicular lymphoma transforming into a Ki-1 lymphoma. Immunogenotyping in this case revealed that the two components were probably of the same clonal origin, as they seemed to share the same light chain gene. The presence of rearrangement in the switch region of the IgH in our case without the actual occurrence of heavy chain switching may have triggered somatic recombination in the IgH complex. This series of events may have led to the transformation of a low-grade lymphoma into a high-grade lymphoma, accounting for the two morphologic patterns seen in our bimorphic lymphoma.